The present invention is directed to a method and apparatus for testing and producing hydrocarbon formations found in deep (600-10,000 feet) offshore waters, and in shallower water depths where appropriate, particularly to a method and system for economically producing relatively small hydrocarbon reserves in mid-range to deep water depths which currently are not economical to produce utilizing conventional technology.
Commercial exploration for oil and gas deposits in U.S. domestic waters, principally the Gulf of Mexico, is moving to deeper waters (over 600 feet) as shallow water reserves are being depleted. Companies must discover large oil and gas fields to justify the large capital expenditure needed to establish commercial production in these water depths. The value of these reserves is further discounted by the long time required to begin production using current high cost and long lead-time designs. As a result, many smaller or "lower tier" offshore fields are deemed to be uneconomical to produce. The economics of these small fields in the mid-range water depths can be significantly enhanced by improving and lowering the capital expenditure of methods and apparatus to produce hydrocarbons from them. It will also have the additional benefit of adding proven reserves to the nation's shrinking oil and gas reserves asset base.
In shallow water depths (up to about 300 feet), in regions where other oil and gas production operations have been established, successful exploration wells drilled by jack-up drilling units are routinely completed and produced. Such completion is often economically attractive because light weight bottom founded structures can be installed to support the surface-piercing conductor pipe left by the jack-up drilling unit and the production equipment and decks installed above the water line, which are used to process the oil and gas produced from the wells. Moreover, in a region where production operations have already been established, available pipeline capacities are relatively close, making pipeline hook-ups economically viable. Furthermore, since platform supported wells in shallow water can be drilled or worked over (maintained) by jack-up rigs, shallow water platforms are not usually designed to support heavy drilling equipment on their decks, unless jack-up rigs go into high demand. This enables the platform designer to make the shallow water platform light weight and low cost, so that smaller reservoirs may be made commercially feasible to produce.
Significant hydrocarbon discoveries in water depths over about 300 feet are typically exploited by means of centralized drilling and production operations that achieve economies of scale. For example, since typical jack-up drilling rigs cannot operate in waters deeper than 300 feet, a platform's deck must be of a size and strength to support and accommodate a standard deck-mounted drilling rig. This can add 300 to 500 tons to the weight of the deck, and even more to the weight of the substructure. Such large structures and the high costs associated with them cannot be justified unless large oil or gas fields with the potential for many wells are discovered.
Depending on geological complexity, the presence of commercially exploitable reserves in water depths of 300 feet or more is verified by a program of drilling and testing one or more exploration and delineation wells. The total period of time from drilling a successful exploration well to first production from a central drilling and producing platform in the mid-range water depths typically ranges from two to five years.
A complete definition of the reservoir and its producing characteristics is not available until the reservoir is produced for an extended period of time, usually one or more years. However, it is necessary to design and construct the production platform and facility before the producing characteristics of the reservoir are precisely defined. This often results in facilities with either excess or insufficient allowance for the number of wells required to efficiently produce the reservoir and excess or insufficient plant capacity at an offshore location where modifications are very costly.
Production and testing systems in deep waters in the past have included converting Mobile Offshore Drilling Units ("MODU's") into production or testing platforms by installing oil and gas processing equipment on their decks. A MODU is not economically possible for early production of less prolific wells due to its high daily cost. Furthermore, now that the market has tightened, such conversions are not considered economical. Similarly, converted tanker early production systems, heretofore used because they were plentiful and cheap, are also not economical for less prolific wells. In addition, environmental concerns (particularly in the U.S. Gulf of Mexico) have reduced the desirability of using tankers for production facilities instead of platforms. Tankers are difficult to keep on station during a storm, and there is always a pollution risk, in addition to the extreme danger of having fired equipment on the deck of a ship that is full of oil or gas liquids. This prohibition is expected to spread to other parts of the world as international offshore oil producing regions become more environmentally sensitive.
Floating hydrocarbon production facilities have been utilized for development of marginally economic discoveries, early production and extended reservoir testing. Floating hydrocarbon production facilities also offer the advantage of being easily moved to another field for additional production work and may be used to obtain early production prior to construction of permanent, bottom founded structures. Floating production facilities have heretofore been used to produce marginal subsea reservoirs which could not otherwise be economically produced. Production from a subsea wellhead to a floating production facility is realized by the use of a substantially neutrally buoyant flexible production riser oriented in a broad arc. The broad arc configuration permits the use of wire line well service tools through the riser system.
FPS (Floating Production System) consists of a semi-submersible floater, riser, catenary mooring system, subsea system, export pipelines, and production facilities. Significant system elements of an FPS do not materially reduce in size and cost with a reduction in number of wells or throughput. Consequently, there are limitations on how well an FPS can adapt to the economic constraints imposed by marginal fields or reservoir testing situations. The cost of the semi-submersible vessel (conversion or new build) and deep water mooring system alone would be prohibitive for most of these applications. In addition, semi-submersibles are now being fully utilized in drilling operations and are not available for conversion into FPS.
A conventional TLP (Tension Leg Platform) consists of a four column semi-submersible floating substructure, multiple vertical tendons attached at each corner, tendon anchors to the seabed, and well risers. A variation of the conventional TLP, a single leg TLP, has four columns and a single tendon/well riser assembly. The conventional TLP deck is supported by four columns that pierce the water plane. These types of TLP's typically bring well(s) to the surface for completion and are meant to support from 20 to 60 wells at a single surface location.
It is therefore an object of the present invention to provide a tension-leg mooring system which suppresses substantially all vertical motions. The mooring configuration of the present invention makes it possible to have a single, stable column piercing the surface of the water with a small water plane area.
It is another object of the invention to provide a tension-leg mooring system having a single surface-piercing column permitting the hull and deck to be independently designed and optimized.
It is another object of the invention to provide a tension-leg mooring system utilizing a foundation having either driven piles, drilled and grouted piles, or suction piles. Redundancy may be incorporated by using a template with additional piles.
It is another object of the invention to provide a tension-leg mooring system wherein the tendons are pre-installed to the foundation and are allowed to float in a more or less vertical configuration until the hull is mobilized to the site and connection to the hull is made.
It is yet another object of the invention is to provide a tension-leg mooring system having a hull which may be wet-towed or dry-towed to the location. After the hull is connected to the pre-installed tendons, the deck sections may be lifted into place.
It is a further object of the invention to provide a tension-leg mooring system wherein the platform has relatively large base dimensions, thereby increasing tendon separation and improving their effectiveness.
It is still another object of the invention is to provide an tension-leg mooring system wherein the key platform components may be standardized.